424 ZOOLOGY 



contain but few species ; the fourth group, however — the 

 Vertebrata — has, from the size of its numerous members and 

 the great variety of form they present, and above all from the 

 fact that it culminates in the genus Homo, acquired a great 

 importance, and it was formerly customary to oppose this 

 group of Vertebrata to the remainder of the animal kingdom, 

 which were classed together under the general heading Inverte- 

 brata. This division of the animal world was to a great extent 

 supported by the fact that much more was known about the 

 anatomy and developement of the comparatively conspicuous 

 Vertebrata than about the much less accessible and often 

 minute Invertebrata, but in late years our knowledge has been 

 much increased in the latter direction, and it is now recognised 

 that the group Vertebrata is, from a strictly morphological 

 standpoint, of no greater interest than any of the other large 

 groups which compose the animal kingdom. 



Class I. Hemichordata. 



Chaeactekistics.— TAe anterior end of the body forms an elon- 

 gated proboscis, at the base of which the mouth opens. The 

 region surrounding and immediately behind the mouth is 

 termed the " collar" and this is succeeded by a trunk con- 

 taining the generative organs. A series of pores or gill- 

 slits are generally present. A diverticulum of the pharynx 

 projects forward into the proboscis-stalk and forms the noto- 

 chord. The sexes are separate. 



The Hemichordata are a group recently constituted by 

 Bateson to contain the genus Balanoglossus. Harmer's investi- 

 gations on the structure of Cephalodiscus have shown that that 

 genus must be assigned a place near Balanoglossus, and still 

 more recently Fowler's researches have shown that Bhdbdo- 

 pleura must also be included in the group. The two last- 

 named forms were previously placed in the neighbourhood of 

 the Polyzoa ; at present we know nothing of their embryology. 

 The group now comprises the three sub-classes (i.) Entero- 

 pneusta, (ii.) Cephalodiscida, (iii.) Ehabdopleurida, each repre- 

 sented by a single genus. 



